One of the key intermolecular interactions triggering critical events in autoimmunity occurs during antigen presentation, when HLA class II alpha and Beta polypeptides bind each other and also bind antigenic peptides. Our goal is to understand the molecular constraints which operate during this interaction and account for the recognition of class II disease susceptibility gene products by specific T cells. We will test the hypothesis that a hierarchy of preferential peptide affinities exists during antigen presentation events, and that such a hierarchy corresponds to genetically determined susceptibility and protection attributed to the HLA gene complex in autoimmunity. We will evaluate the ability of a single peptide antigen to preferentially bind different class II molecules in a competitive environment, namely intracellular endosomal compartments containing multiple class II dimers. We will then test the molecular mechanism by which polymorphic variation in class II molecules alters the peptide binding interaction in this environment, in order to pinpoint the contribution of specific genetic elements to competitive events in immune activation. Class II alpha and beta cDNA, with selected introduced mutations, will be expressed in class II-negative B-LCL using retroviral vectors; these cells will be tested for ability to present peptides to cloned T cells under conditions designed to test for class II intermolecular competition for peptide binding. These studies will identify and characterize and important step in the autoimmune response pathway, a step suitable for novel therapeutic intervention strategies.